Group of pictures (GOP) configuration
Description
GOP parameters define the basic pattern of the video stream in terms of how the encoder uses I-, P-, and B- frames.
The parameters that control the GOP structure are the following:
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GOP Mode: Choose Fixed.
Recommendation: Always choose Fixed. The Follow mode is obsolete and is not recommended.
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GOP Size: Defines the interval between I-frames.
Recommendation: When using MPEG-2, the recommended GOP size is up to 30. A size of 15 is also common.
For H.264, the recommendation is to make the GOP size as large as possible while still meeting other encoding requirements. For example, for adaptive bitrate delivery in which a segment size of 6 seconds is used for 29.97 fps outputs, the largest GOP size should be 180 frames.
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B-Frames: Defines the maximum run of B-frames. Note that the encoder might sometimes decide to use a smaller run of B- frames within the GOP. It usually does this to produce higher video quality.
Recommendation: When using H.264 or H.265, enable GOP Reference B-Frame to obtain the best quality and set B-frames to a value from 3 to 5 (3 is recommended).
For other codecs, there is no quality benefit to setting the B-frames to more than 2. For high-motion content, use 0 or 1 to produce the best quality.
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Closed GOP Cadence: Defines the number of GOPs across which P- or B-frames are allowed to predict for encoding purposes.
Recommendation: Set the parameter to 1 for segmented outputs such as adaptive bitrate content in HLS or DASH outputs,
Set the parameter to 0 for non-segmented outputs, to allow for an open GOP.
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GOP Reference B-Frame (H.264 and H.265 only): Enables the use of reference B-frames for GOP structures that have B- frames greater than 1.
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Min I-interval: Specifies a minimum number of frames between I-frames for GOP cadence and I-frames for scene change detection. I-frames require more bits than P- or B-frames, so encoding two in quick succession can hurt quality, particularly with small buffer sizes. The encoder enforces the minimum I-interval by shifting the GOP cadence.
Recommendation: For segmented outputs that require a fixed cadence of I-frames, set this parameter to 0, in order to disable it.
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Reference Frames: Applies only to H.265. This parameter defines the number of frames that can be used to define B- and P- frames.
Recommendation: We recommend that you set this parameter to 3.
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Scene Change Detect: Enables an algorithm that determines when a scene change occurs. The encoder inserts an I-frame at each scene change.
Recommendation: Always enable this detection for the best video quality. The only scenario where you might disable this detection is to accommodate the rare case that a set-top box or playback device can't accommodate an additional I-frame within the normal GOP pattern.
Some service providers, encoding vendors, or manufacturers might state that scene change detection should be disabled during encoding of content. Typically, they make this recommendation because their systems require a consistent GOP structure. They assume that the encoder resets the GOP structure when an I-frame is inserted for a scene change, which disrupts the GOP structure.
However, the encoder doesn't disrupt the GOP structure in this way. Instead, the encoder inserts an additional I-frame on a scene change. With this approach, the adaptive bitrate outputs are always GOP-aligned. This alignment has the side effect of allowing for compatibility with these third-party systems, even when scene change detection is enabled.
Location of parameters
This table shows where the parameters mentioned in this section are located. The first column shows the location on the web interface. The second column shows the location in the event XML.
| Location of parameter on web interface | Location of tag in XML |
|---|---|
| Streams – Video > Advanced > GOP Size |
stream_assembly/video_description/ where codec is one of the following:
|
| Streams – Video > Advanced > B Frames |
stream_assembly/video_description/ gop_num_b_frames where
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| Streams – Video > Advanced > Closed GOP Cadence |
stream_assembly/video_description/ gop_closed_cadence where
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| Streams – Video > Advanced > Reference Frames |
stream_assembly/video_description/h265_settings/num_ref_frames |
| Streams – Video > Advanced > Scene Change Detect |
stream_assembly/video_description/ transition_detection where
|
| Streams – Video > Advanced > Min I-interval |
stream_assembly/video_description/ min_i_interval where
|
| Streams – Video > Advanced > GOP Reference B-Frame |
stream_assembly/video_description/ gop_b_reference where
|
